Electrical workers and others working nearby an electrical structure such as overhead power lines, underground cables or switching cubicles can get in contact with an electrically energized object accidentally. This object can introduce a dangerously high voltage to the body of the worker. If the worker stands on the ground, the voltage difference (i.e. voltage potential) between the contact point and the worker's foot can drive a large current flowing through his/her body, leading to electrocution. One of the effective ways to mitigate such a risk is to create an equipotential (equal-voltage-potential) zone for the worker, as illustrated in FIG. 1. Working inside this zone, the worker will not experience the voltage potential between any parts of his body and, as a result, no or little current can flow through the body. Therefore, equipotential zone is a very important means to create safe work environment for such workers.
There is a few ways to create such an equipotential zone. For example, the worksite can be covered by a metal plate for the worker to stand on. The plate is bonded to any structure that could be energized and be contacted by the worker. However, such an approach is not practical as it imposes many constraints to the workers. Furthermore, a large metal plate that can cover the worksite or a large part of it lacks portability. What industry needs is a device that is portable and can create an equipotential zone for a wide variety of worksites and working conditions.
In response to this need, U.S. Pat. No. 6,477,027B1 and U.S. Pat. No. 5,835,332 presented a portable equipotential mat. It comprises a flexible base having a conductive mesh attached to a surface of the base. A preferred material for the base is a vinyl/polyester fabric and the conductive mesh is manufactured from high ampacity tinned copper braid. In recent years, the effectiveness of such an equipotential mat has been questioned by industry. It is clear that the conductive mesh cannot create an equipotential zone since there are open spaces inside the mesh. If a worker's foot happens to step on the space instead of the conductive braid, the foot will be in ground potential which is different from the potential of the conductive braid when the mat is energized. Note that the base is not treated as an insulating material and has not been claimed as dielectric in the patents. The issue here is if the potential difference is small enough so that the area covered by the mat can still be considered as an equipotential zone for practical purposes. Unfortunately, research results have shown that the potential difference can be huge, such as 8 kilovolts when the mat is energized by a 25 kilovolt source. In addition, the mat has other problems. One of them is that the braid can be easily worn out which limits the conductiveness of the mesh. Another is that it is very difficult to inspect the integrity of the mat from maintenance perspective. As a result of these findings, industry is looking for alternative methods or devices to create portable equipotential zone.